I wish I would have taken more photo's so I could share this idea better by writing a tutorial for you. But I was 8 months pregnant with twins, so uncomfortable and I just wanted to get these finished before the babies were born. Plus, different brands of car seats have different placement of the straps inside.
I based the blankets off of a stroller blanket pattern that I purchased from MyTaylorMade on Etsy. The pattern can be found here. The pattern has a "boot" for the babies legs and feet and the two sides fold in and cover the baby.
The outside is cotton fabric and the inside is minky. I am glad I chose minky, it is just so soft. I know that the babies love to be snuggled down in their seats and I believe that the minky fabric is the reason why. It covers all the rough and scratchy car seat fabric and gives it a nice luxurious feel.
I would love to hear what you think and see pictures if you make some!